Nonlocal observables and lightcone-averaging in relativistic thermodynamics

TL;DR

This paper proposes a lightcone-based approach to relativistic thermodynamics, resolving longstanding ambiguities by defining thermodynamic quantities relative to an observation event's backward lightcone, supported by simulations.

Contribution

It introduces a novel lightcone-averaging method for relativistic thermodynamics, enabling consistent definitions of heat and work and extending the framework to General Relativity.

Findings

Lightcone-averaging yields testable predictions.

Simulations confirm the consistency of the approach.

Framework extends to General Relativity.

Abstract

The unification of relativity and thermodynamics has been a subject of considerable debate over the last 100 years. The reasons for this are twofold: (i) Thermodynamic variables are nonlocal quantities and, thus, single out a preferred class of hyperplanes in spacetime. (ii) There exist different, seemingly equally plausible ways of defining heat and work in relativistic systems. These ambiguities led, for example, to various proposals for the Lorentz transformation law of temperature. Traditional 'isochronous' formulations of relativistic thermodynamics are neither theoretically satisfactory nor experimentally feasible. Here, we demonstrate how these deficiencies can be resolved by defining thermodynamic quantities with respect to the backward-lightcone of an observation event. This approach yields novel, testable predictions and allows for a straightforward-extension of thermodynamics to General Relativity. Our theoretical considerations are illustrated through three-dimensional relativistic many-body simulations.